This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 00 15981 filed in France on Dec. 8, 2000; the entire content of which is hereby incorporated by reference.
The invention relates to a sandwich acoustic panel, in other words a noise reducing sandwich panel designed to attenuate an incident sound wave facing an outside face of the panel.
In particular, an acoustic panel according to the invention may be used in the walls of pods or turbojet casings, or in ducts to be soundproofed, etc.
Existing acoustic panels usually comprise one or several quarter wave resonators superposed on a total reflector. Each resonator itself is composed of a resistive layer that is more or less permeable to air, and a compartmentalized structure, usually of the honeycomb type. The resistive layer covers the face of the compartmentalized structure facing outside, in other words towards the incident sound wave. On the other hand, the total reflector covers the face of the resonator opposite this incident wave. By convention, the xe2x80x9cfront facexe2x80x9d is the side of the panel on which the resistive layer is placed, and the xe2x80x9cback facexe2x80x9d is the opposite side of the panel covered by the reflector.
In this conventional arrangement of acoustic panels, the resistive layer performs a dissipation role. When a sound wave passes through it, viscous effects occur that transform the acoustic energy into heat.
The thickness of the compartmentalized structure can be varied to match the panel to the characteristic frequency of the noise to be attenuated. The noise dissipation in this resistive layer is maximum when the height of the cells in the compartmentalized core is equal to a quarter of the wavelength of the frequency of the noise to be attenuated. Cells in the compartmentalized structure then behave like wave guides perpendicular to the surface of the panel, such that they have a xe2x80x9clocalized reactionxe2x80x9d type response. The cells form an assembly of quarter wave resonators in parallel.
The back reflector creates total reflection conditions essential for the behaviour of the compartmentalized core described above.
In general, an acoustic panel must satisfy acoustic requirements.
The first of these requirements applies to the acoustic homogeneity of the panel. In other words, the acoustic processing is particularly effective if it is conform with its specification over its entire area. Failure to respect this requirement depends on the nature of the elements making up the panel, their relative layout and adhesives used for their assembly.
Another acoustic requirement is the xe2x80x9clocalized reactionxe2x80x9d requirement. If this requirement is not satisfied, then there is a transverse propagation of sound waves called xe2x80x9clateral energy leakxe2x80x9d inside the panel, which opposes xe2x80x9cquarter wavexe2x80x9d type operation of the compartmentalized structure.
When the panel is fitted on an aircraft engine, these acoustic requirements are combined with other requirements for resistance to the environment, structural requirements and aerodynamic requirements.
Thus, an acoustic panel integrated in an aircraft engine must be able to resist severe usage conditions. In particular, the panel must not become delaminated, even in the presence of high negative pressures, it must be capable of resisting corrosion and erosion, for example due to sand, and it must have a good electrical conductivity particularly in order to resist lightning strikes and it must contribute to the mechanical absorption of shocks following the loss of a blade.
An acoustic panel integrated in an aircraft engine must also have sufficient structural strength to resist the weight of a man and to transfer aerodynamic and inertial forces from the air intake to the engine casing.
Finally, the surface condition of an acoustic panel integrated in an aircraft engine must be consistent with the aerodynamic lines and continuity requirements of surfaces in contact with air flows.
Known acoustic panels may be classified in three categories; panels with a non-linear single degree of freedom (non-linear SDOF), panels with a linear single degree of freedom (linear SDOF), and panels with two degrees of freedom (double degree of freedom (DDOF)).
In panels with a non-linear single degree of freedom, the resistive layer is composed of a perforated metallic or composite layer.
The advantage of a panel of this type is that it enables good control over the percent of open surface area, it has good structural strength and is easy to make.
On the other hand, it has the disadvantage that it is acoustically very non-linear and that the strength is very dependent on the tangential flow velocity at the surface. Furthermore, since the frequency damped by each cell depends on its depth, and since the depth of all cells in the panel is the same, the frequency range damped by this type of panel is restricted. Furthermore, when the resistive layer is made of a composite material, the structure has low resistance to erosion.
In acoustic panels with a linear single degree of freedom, the resistive layer is a micro-porous layer, for example composed of a metallic fabric, a perforated plate combined with an acoustic fabric or a metallic fabric associated with an acoustic fabric.
The use of this type of panel makes it possible to adjust the acoustic resistance by modifying the components of the micro-porous layer. It is efficient over a reasonable frequency range. This type of panel also has the advantage that its non-linearity is low to moderate, while the acoustic resistance is only slightly dependent on the tangential flow speed at the surface.
However, the production of a sandwich panel with a linear single degree of freedom is more complicated than the construction of a panel with a non-linear single degree of freedom, since the resistive layer comprises two components. If the components or assembly processes are not controlled, the structure may comprise areas of acoustic non-homogeneity, or risks of delamination of the resistive layer. Furthermore, risks of corrosion in the resistive layer impose an additional constraint on the choice of the material used. Furthermore, the process for assembly of this type of panel is long and expensive.
Finally, an acoustic panel with two degrees of freedom comprises two superposed compartmentalized cores, in addition to a perforated resistive layer and a back reflector, separated by an intermediate resistive layer called the xe2x80x9cseptumxe2x80x9d which is usually micro-porous.
Compared with the other types of acoustic panels, panels with two degrees of freedom have a wider damped frequency range, a possibility of adjusting the acoustic resistance by means of two resistive layers, and low to moderate acoustic non-linearity.
However, acoustic panels with two degrees of freedom have the disadvantage that areas of acoustic non-homogeneity occur due to poor alignment of the cells in the two compartmentalized cores, that inevitably occurs when the panel is being formed. There are also parasite transverse propagation phenomena in areas in which the cells of the two compartmentalized cores are not aligned. Finally, the process for assembly of a panel of this type is long and expensive, since the various elements of the structure have to assembled one by one.
The purpose of the invention is an acoustic panel with an innovative design that would enable it to take advantage of panels with several degrees of freedom, while eliminating the disadvantages due to alignment defects in the cells of compartmentalized structures, such as the risks of acoustic non-homogeneity and transverse propagation of acoustic waves.
According to the invention, this result is achieved by means of a sandwich acoustic panel comprising a resistive layer forming a front face of the panel, a compartmentalized structure formed from at least two superposed compartmentalized layers each comprising a network of cells, a porous separator inserted between the adjacent compartmentalized layers and a reflector forming the back face of the panel, characterized in that the porous separator is provided with guides on each face penetrating into at least some of the cells of the compartmentalized layers adjacent to the separator, distributed over the entire surface of the separator.
The presence of guides on each face of the porous separator makes it possible for partitions, and consequently cells of the compartmentalized structure, to be made continuous between the inner surface of the resistive layer and the reflector. Therefore local misalignment problems of cells that necessarily occur on panels with several degrees of freedom according to prior art, composed of several superposed compartmentalized structures, are eliminated. Consequently, risks of non-homogeneity no longer exist.
According to one preferred embodiment of the invention, the resistive layer, compartmentalized layers, the porous separator and the reflector are assembled to each other by bonding.
Advantageously, the resistive layer, the compartmentalized layers, the porous separator and the reflector are all made from identical materials or materials compatible with the adhesive used to assemble them.
These materials are preferably chosen from the group comprising metallic, composite and thermoplastic materials.
Depending on the case, guides include either aligned elements, positioned on each side of the porous separator, or elements passing through the porous separator.
In the preferred embodiments of the invention, the guides are tubular or formed of solid rods, of circular cross-section. This cross-section may be substantially uniform over the entire length of the guide or, on the contrary, provided with tapered ends in order to improve their mounting. They may have a different shape, for example a star-shaped section with at least three branches, without going outside the scope of the invention. In addition, the rods may be made from a porous material or not.